With 158 delegates at stake, Pennsylvania offered the largest prize remaining in a primary season that ends on June 3.

The Clinton campaign hoped for a sizable win that would strengthen her claim to being the stronger general election opponent, an argument she has made to Democratic officeholders and other superdelegates who hold the balance of power at the party convention in Denver in August.

But the final outcome may be closer than Clinton had hoped.

"Right now there is real concern inside the Clinton camp that this will be a lot closer than they wanted," reports CBS News chief White House correspondent Jim Axelrod.

CBS News early exit polls show that most Pennsylvania Democrats made up their minds a long time ago, while only 23 percent decided within the last week.

But voters who made late decisions broke to Clinton, with those deciding in the last week supporting the New York senator 58 percent to 42 percent. Voters who decided in the last three days also backed Clinton 60 percent to 40 percent for Obama.

New Democratic voters, who either switched from another party or registered as a Democrat for the first time, strongly backed Obama at a rate of 60 percent to 38 percent.

Women made up 58 percent of Pennsylvania Democratic voters, and they voted for Clinton over Obama 55 percent to 44 percent. Obama won a majority of men (53 percent).

Clinton won the support of 60 percent of white voters, while Obama was the overwhelming choice of black voters (92 percent). Clinton won the crucial demographic of white men, garnering 55 percent of their vote and also took 64 percent from blue-collar white men.

As has been the case in other states, the economy was the most important issue to voters, with 54 percent of Pennsylvania Democrats describing it as such, and most Democrats saying that the economy is in a recession. The war in Iraq and health care were the other top issues to voters.

Voters' loyalty to their chosen candidate was high, as more than six in ten (64 percent) Clinton voters said they would not be satisfied if Obama ended up the Democratic nominee, and more than half of Obama voters (54 percent) said they would not be satisfied if Clinton won the nomination. Overall, 70 percent of Pennsylvania Democrats would be satisfied if Clinton were the nominee, while 64 percent would be satisfied with Obama.

Obama began the night with a delegate lead of 1,664-1,531 in the latest CBS News count, out of 2,025 needed to win the nomination. (Click here for state-by-state tallies.)

"That Obama will be traveling to Indiana, the site of the next important primary contest, tonight tells you his campaign isn’t banking on basking in the glow of an upset win in the Keystone State," said CBSNews.com senior political editor Vaughn Ververs. "And whether Clinton wins by one point or 15 points, her campaign has given no indication that she’s even considered exiting the race after a win in Pennsylvania." (Read more from Ververs in CBSNews.com's campaign blog Horserace.)

The six-week run-up to the primary was notable for close-to-the-ground campaigning normally reserved for the early-voting states of Iowa and New Hampshire, and for the decidedly negative tone of its final few days.

Flush with cash, Obama reported spending $11.2 million on television in the state, compared with $4.8 million for Clinton. (See the latest campaign finance reports.)

The tone of the campaign was increasingly personal.

"In the last 10 years Barack Obama has taken almost $2 million from lobbyists, corporations and PACs. The head of his New Hampshire campaign is a drug company lobbyist, in Indiana an energy lobbyist, a casino lobbyist in Nevada," said a Clinton commercial that aired in the final days of the race.

Obama responded with an ad that accused Clinton of "eleventh-hour smears paid for by lobbyist money." It said that unlike his rival, he "doesn't take money from special interest PACs or Washington lobbyists - not one dime."

To the delight of Republicans, the six-week layoff between primaries produced a string of troubles for the Democrats.

Obama was forced onto the defensive by incendiary comments by his pastor, Rev. Jeremiah Wright, then triggered controversy on his own by saying small-town Americans cling to guns and religion because of their economic hardships.

Clinton conceded that she had not landed under sniper fire in Bosnia while first lady, even though she said several times that she had. And she replaced her chief strategist, Mark Penn, after he met with officials of the Colombian government seeking passage of a free trade agreement that she opposes.

John McCain, the Republican nomination already his, rose in the polls as he prepared for the fall campaign.

The CBS News early exit polls show that the majority of Pennsylvania Democrats said they would rally behind whoever becomes the Democratic nominee. They would back Obama over McCain (72 percent to 15 percent) and would support Clinton over McCain by an even wider margin (80 percent to 11 percent).

Twenty-six percent of Clinton voters in the Democratic primary say they would support McCain in the general election, while 50 percent would back Obama. Among Obama voters, 17 percent say they would vote for McCain, while 67 percent would side with Clinton.

The remaining Democratic contests are primaries in North Carolina, Indiana, Oregon, Kentucky, West Virginia, Montana, South Dakota and Puerto Rico, and caucuses in Guam.

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